The EU plans to have them by the end of summer to allow in-bloc travel. The US has ruled them out on a federal level. And in Israel, vaccinated people are already using an app to enter gyms, pools and cultural venues.

So-called vaccine passports have been hotly debated around the world. These certificates proving one has been immunized against COVID-19 are designed to grant safe access to certain businesses and international travel.

While critics fear segregation between vaccinated people and those who can't or won't get jabbed, many supporters point out that showing proof of immunization is already quite common, especially for travelers and migrants.

No vaccine? No immigration for some

When former US representative Justin Amish tweeted that it was "dystopian" to carry around health records, and American media personality and physician Dr. Drew Pinsky asked what people's reaction would be if international travel required other vaccinations as well, both faced an immediate backlash.

Social media users pointed out that many countries already require certain vaccines and health papers.

People who want to immigrate to the US need to show proof of 14 vaccines and pass a medical exam that includes tuberculosis and syphilis tests. Countries like Ghana and Uganda demand proof of a yellow fever vaccine from travelers and, in Saudi Arabia, one can't do the pilgrimage to Mecca without being immunized against meningitis.

Some countries also have vaccine mandates to stop the spread of infectious diseases from childhood.

In Italy, parents risk a fine if their children go to school without being immunized against 12 preventable diseases. Germany made the measles vaccine compulsory for kids enrolling in daycare or school. And Australia says "no jab, no pay" to parents who want to receive certain state benefits when their children haven't been fully immunized.

'You only want to hold on to your freedom'

Luiza Pelucio, a 25-year-old consultant in Chicago, is all too familiar with providing immunization records. When she moved to the US from Brazil in her teens, she had to receive about seven shots to meet the public school requirements in New York and to qualify for a green card.

Once all adults are eligible for the COVID-19 vaccine nationwide, Pelucio hopes vaccine certificates will be necessary to enter certain places of leisure.

"I always thought it was valid that they required me to get these vaccines to move to the US because it's about protecting society," she said. "I think it's the same concept now during the pandemic. It's just making sure your citizens are safe." 

While Rohan Moona, a university student from India, didn't have to show his immunization records to get a UK visa earlier this year, he did have to take a tuberculosis test. He feels that, among people opposing vaccine passports, there is a double standard about who is and who isn't expected to show their health records.

"You're potentially taking away my freedom to explore a country, but you're not willing to show a vaccine document because you only want to hold on to your own freedom," Moona said.

Conditioning social behavior

The main difference between the vaccine mandates that some countries already have and the vaccine passports that are being discussed now is, of course, that the COVID-19 vaccine certificate would be more far-reaching.

Besides travel and school enrollment, it could affect leisure activities in your country of residence — from bars to restaurants to concerts.

But for Dr. Joan Costa Font, a professor of health economics at the London School of Economics, requiring vaccine passports for non-essential activities isn't a big jump. He believes it is just like any other way that a government sets signals to change society's behavior.

"Vaccine passports are a conditioning mechanism that we use to incentivize people to engage in prosocial behavior," he said. "It's like paying taxes. If you don't pay taxes, you're not being social with the other individuals in society."

Not everyone has a choice

The main problem, Dr. Costa-Font says, is vaccine inequity.

There is a massive disparity between the vaccine rollout in wealthier and poorer nations. Even within wealthier countries, not everyone has the same access, with Black and Hispanic people in the US getting smaller shares of the vaccine, for example.

Of course, some people can't get vaccinated for health reasons, like pregnant women or people who are allergic to substances in the vaccine.

But  Dr. Costa-Font maintains that as long as all adults are eligible to get vaccinated in a country and exceptions are made for those who can't, the benefits of vaccine passports outweigh the negatives, as the main goal is to incentivize people to curb the spread of the coronavirus.

"You're not just protecting yourself. You're protecting others," said Dr. Costa-Font. "By not taking the vaccine, you're depriving others of those beneficial effects."